1. Field of the Invention
The present invention generally relates to optical disc apparatuses and methods of recording and reproducing data.
2. Description of the Related Art
In CD-RW media, reversible transitions between a crystalline phase and an amorphous phase, which have respective reflective indexes, achieve the recording of information. Whether the recording film turns into the crystalline phase or the amorphous phase depends on the speed of cooling after the application of heat. Slow cooling provides a crystalline phase, and rapid cooling achieves an amorphous phase.
In practice, a laser beam having predetermined erase power Pe is scanned to create a crystalline phase (space). A laser beam having large write power Pw and small bottom power Pb, which are repeated at short intervals, is scanned to create an amorphous phase (mark). A laser beam having read power Pr is shone on a recorded CD-RW disc to read amorphous phase marks.
Conventionally, sufficient record quality is achieved even if the bottom power Pb and the read power Pr are the same.
Because of the recent improvement of record films used in the CD-RW media, speed at which information is properly recorded on the CD-RW media has been increasing. With such an increase in recording speed, a power margin has been decreasing, resulting in difficulties in providing stable record quality when compared with the use of a slower recording speed.
In the following, technologies relating to a related-art optical disc apparatus will be described.
<Technology 1>
For every 2T increase in the mark data length, the number of heating pulses and the number of cooling pulses are increased by one, respectively. This achieves sufficient heating and cooling even at high recording speed (Japanese Patent Application Publication No. 2002-334433).
<Technology 2>
The laser diode exhibits a linear relationship between an electric current and the power of light emission when the electric current is sufficiently large to generate emission power exceeding a predetermined level. This makes it easier to select the erase power Pe and the write power Pw.
Nowadays, many notebook-type personal computers are equipped with a CD-R drive. Since notebook-type personal computers are typically driven by use of a buttery, the CD-R drive provided as a peripheral needs to be designed to run with small power.
In the recording of CD-R media, pits are created by severing a record film by heat. In practice, a laser beam having the read power Pr is applied during the scan for creating a space, and a laser beam having the write power Pw is applied during the period for creating a mark.
<Technology 3>
In a large number of existing laser drivers, maximum tolerable electric currents are different for respective channels. Each channel has its specific usage such as the read purpose or the write purpose, and the design of an electric current is made to conform to the respective usage. A recording apparatus having three channels, for example, may be designed such that the maximum tolerable current is 50 mA for channel 1, 150 mA for channel 2, and 100 mA for channel 3. In the case of CD-R media, for example, the channel 1 is used for producing read power, the channel 2 for producing write power, and the channel 3 for producing peak power. In the case of CD-RW, for example, the channel 1 is used for producing base power, the channel 2 for producing erase power, and the channel 3 for producing write power.
In related-art optical disc apparatuses, the speed of recording CD-RW. media is slower than the speed of recording CD-R media. Because of this, the record power would not be perceived as lacking sufficient power.
The technologies relating to the related-art optical disk apparatuses described above have drawbacks as follows.
<Problem 1>
A laser diode used in the related-art optical disc apparatuses for recording CD-RW media does not exhibit a linear relationship between an electric current and the power of light emission in the region where the light emitting power is small. If the erase power Pe is excessively reduced, thus, a problem will be encountered in selecting the erase power Pe and the write power Pw.
Accordingly, there is a need for a scheme that improves record quality in high-speed recording without affecting the write power and the erase power.
<Problem 2>
In general, the power of light emission of a laser diode is set to a small level for the read power Pr. However, the relationship between an electric current and the power of light emission has such characteristics that large current is consumed even for the small level of the read power Pr. This results in the requirement of an excessive current despite the fact that relatively small emission power is necessary.
Accordingly, there is a need for a scheme that reduces current consumption at the time of recording by suppressing current consumption in the region where a space is created.
<Problem 3>
It appears that the recording speed of CD-R media has recently been reaching its maximum limit, whereas the recording speed of CD-RW media is yet increasing. This gives rise to a problem in that sufficient recording power cannot be attained for the recording of CD-RW media. For example, a current of 80 mA may be necessary as erase power, and a current of 120 mA may be necessary as write power. In such a case, a problem is encountered in that the current of the channel 3 exceeds its tolerable current limit.
The setting of emission power, i.e., a setting made to a current source for driving a laser diode, is generally done by a DA converter. When the maximum level of emission power and the resolution of settings are taken into consideration, the use of a 10-bit DA converter is generally required. Because of the large chip size of a DA converter, it is desirable to reduce the number of bits required for the DA converter.
Accordingly, there is a need for a scheme that broadens the range of power settings in response to a required amount of a current even when each current source is designed for a respective current amount corresponding to its predetermined usage.